The present invention relates to an engine ignition timing control system and a method which are designed to utilize a speed density system for calculating the quantity of intake air in the combustion chamber of an internal combustion engine on the basis of pressure data in the intake manifold of the internal combustion engine and which have variable cylinder operation modes.
The control system of the conventional internal combustion engine is so constructed that many operation data of the engine are gathered by many sensors for calculating predetermined control values in response to the operation data with suitable calculating means so that many actuators are driven by output signals responsive to the calculated control values to enable many mechanisms to be controllably driven in response to the predetermined control values.
In the conventional internal combustion engine, the quantity of intake air (A/N) to be supplied to the combustion chamber is adjusted in response to the opening of a throttle valve, and the quantity of fuel corresponding to the quantity of intake air in response to the opening of the throttle valve and the revolution of the engine is supplied to the combustion chamber of the engine so as to perform an ignition process at an adequate ignition timing in response to the quantity of intake air and the revolution of the engine.
There has been well known in the art a typical engine which employs an ignition timing control system called "a speed density system" for calculating intake air quantity data for use in a fuel supply mechanism and advanced angle quantity data for use in ignition timing control on the basis of pressure levels of air in the intake manifold. The speed density system is advantageous in that a pressure sensor is provided in an air duct held in communication with the intake manifold to sense pressure levels of air in the intake manifold by way of the air duct instead of an airflow sensor provided in the intake manifold to directly sense the quantity of intake air in the intake manifold, resulting in reducing the intake air resistance of the intake manifold and thus in decreasing costs of the sensors.
On the other hand, the internal combustion engine having such a speed density system is operated in such a manner that a map is prepared for a standard ignition timing .theta.b in response to an intake manifold pressure Pb in place of an intake air quantity A/N and an engine revolution Ne upon calculation of the ignition timing of the engine. In such an internal combustion engine, the standard ignition timing .theta.b is initially calculated in response to the engine revolution Ne and the intake manifold pressure Pb, and then compensated on the basis of coolant temperature Twt, intake air temperature Ta and the like for the purpose of calculating a target ignition timing .theta.adv.
In the conventional speed density system, the intake manifold pressure including the intake air quantity data is sensed for calculation of the intake air quantity and the ignition timing of the engine so that the operation mode variation of each of the cylinders of the engine such as, for example, the variation of the valve opening and closing timing caused by high and low cams of the intake and exhaust mechanism, and the variation of the intake air quantity caused by switching all cylinders operating conditions and partial cylinders operating conditions in which the intake and exhaust mechanisms of the cylinders are partially held in their non-operating condition. This results in the fact that the intake air pressure, the intake air quantity and the ignition timing of the engine are not associated with each other, thereby reducing desirable levels of the calculated intake air quantity and ignition timing of the engine.